Prior Art
Presently there is considerable attention being paid to methods by which a laser-electrophotographic printer/copier terminal can be used to scan a document and convert information on a document into electrical signals. A terminal with such a scanning capability can be used as a high performance facsimile terminal. Two approaches to this objective are to use the laser beam existing in the terminal to sense the document image either by scanning the original or by scanning the toned image on the photoconducting drum. Both of these schemes have serious drawbacs. For example, the former scheme lacks panchromatic response and requires cumbersome optics. In addition, as a result of the potential health hazard involved since the scanning laser beam is directed up and toward the original document, this approach requires awkward covers. The latter scheme is fundamentally awkward in that both the photoconductor and the toner particles are absorbing at the wavelength of the printing laser so that contrast is very poor and the resulting signal difficult to process. Multi-wavelength lasers would solve the problem but are expensive and not sufficiently reliable.
Additionally, the development of a printer/copier facsimile transmitting terminal has been discouraged because of the long time it takes for a laser to discharge the surface of the photoconductor. For example, in order for the facsimile transmitting capabilities to be practical, the photoconductor must be completely scanned by a laser beam within several seconds. At this scan speed, the laser dwells on each picture element for only a fraction of a microsecond. In such a small time, it is difficult to sufficiently discharge the photoconductor to produce a useable signal, because of the finite time required for laser excited charge to transit the photoconductor.